1. Field of the Invention
The present invention relates to a structure of a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element such as an insulated-gate type transistor, and to a method of manufacturing the same. In particular, the present invention relates to a structure of a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element having the LDD structure formed with the use of a resist, and a method of manufacturing the same. A semiconductor device according to the present invention includes not only an element such as a thin film transistor (TFT) or a MOS transistor but also an electro-optical device such as a display device or an image sensor which has a semiconductor circuit consisting of the insulated-gate type transistor. In addition, a semiconductor device of the present invention also includes an electronic equipment provided with those electro-optical devices.
2. Description of the Related Art
What have attracted attention is an active matrix type liquid crystal display device in which a pixel matrix circuit and a driver circuit consist of a thin film transistor formed on an insulating substrate. A liquid crystal display of about 0.5 to 20 inches in size is utilized as a display device.
A TFT having as an active layer a crystalline semiconductor film, typical example of which is a polysilicon film, at present receives attention in an attempt to realize a liquid crystal display capable of displaying with high definition. However, the TFT having as the active layer the crystalline semiconductor film is, on one hand, higher in the operation speed and driving performance as compared with a TFT having as an active layer an amorphous semiconductor film, but, on the other hand, has a problem of significant fluctuation in characteristics between one TFT and another.
As one of the factors in this fluctuation in TFT characteristics, the interface between the active layer and the gate insulating film may be named. When contaminated, this interface affects the TFT characteristics. It is therefore important to purify the interface between an active layer and an insulating film that is in contact with the active layer.
Sought in a TFT now is high mobility, and the potential for use as an active layer of a TFT is a crystalline silicon film that has higher mobility than an amorphous silicon film. An outline of a conventional method of manufacturing a TFT will be described below in a simplified manner.
First, a gate wiring is formed on an insulating substrate, and a gate insulating film and an amorphous silicon film are layered thereon to form a polysilicon film by applying on this amorphous silicon film a crystallizing process such as heating or laser light irradiation. Subsequently, this polysilicon film is patterned into a desired shape to form an active layer. The polysilicon film is then selectively added with impurities that give P type or N type conductivity to form an impurity region to be a source region or a drain region. An interlayer insulating film is subsequently formed by deposition, a contact hole is formed to expose the source region or the drain region and, thereafter, a metal film is formed and patterned to form a metal wiring that is brought into contact with the source region or the drain region. The manufacturing process of a TFT is thus completed.
As is described, conventionally, an amorphous semiconductor film is exposed to the air because an insulating film is not formed until several steps (a crystallizing step and a patterning step, for example) are applied after formation of the amorphous semiconductor film.
The air within a clean room, in particular, contains mainly boron escaped from an HEPA filter that is commonly used for purifying, and boron is mixed in the active layer in an uncertain amount when the active layer is exposed to the air. In a conventional case where the active layer is fabricated being exposed to the air, the SIMS analyzation shows that the concentration of boron peaks (the concentration peak is shown by the dotted line B in FIG. 15) at the interface (on the main front surface side or on the back surface side) of the active layer of a TFT, and its maximum value reaches 1xc3x971018 atoms/cm3 or more. When boron is mixed in the active layer as above, the concentration of impurities in the active layer is difficult to control and the mixed boron causes the threshold of the TFT to vary. To manufacture the layer with the use of other filter costs, and hence is not a proper solution for the problem.
As described above, conventionally, a semiconductor film is formed only to expose its surface to the air, thereby contaminating itself, i.e. the semiconductor layer to be an active layer, with impurities in the air (boron, oxygen, moisture, sodium, etc.). Otherwise, a gate insulating film is formed and then contaminated from the exposure to the air and, in turn, it contaminates with impurities in the air (boron, oxygen, moisture, sodium, etc.) a semiconductor film that is formed to be an active layer on the gate insulating film. When a semiconductor element, for example, a TFT is manufactured using thus contaminated semiconductor film, characteristics of the interface between the active layer, in particular, a channel formation region and the gate insulating film are degraded, causing fluctuation and degradation in electrical characteristics of the TFT. Also, the impurities (boron, oxygen, moisture, sodium, etc.,) inhibit crystallization of the semiconductor film at the crystallization step.
An object of the present invention is to provide a semiconductor device and a method of manufacturing the same, the device being provided with a semiconductor circuit consisting of a semiconductor element that is capable of improving characteristics of a TFT and has uniform characteristics, the device and the method being provided by improving the interface between an active layer, in particular, a region for constructing a channel formation region and an insulating film.
The structure of a thin film transistor in which an LDD region is provided is conventionally known. An example of the thin film transistor provided with the LDD region is disclosed in Japanese Patent Application Examined No. Hei 3-38755 and in Japanese Patent Application Laid-open No. Hei 7-226515. The LDD region serves to temper the strength of the electric field formed between a channel formation region and a drain region to prevent decrease in OFF current in and degradation of the transistor. However, a manufacturing method of the LDD structure using a conventional technique is complicated and requires many steps.
The present invention further has an object to provide a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element that includes an LDD structure with high productivity, which is high in reproducibility and capable of improving stability of transistor characteristics, and to provide a method of manufacturing the semiconductor device.
To attain the above objects, the present invention is given one feature, among other features, that a gate insulating film and a semiconductor film are formed without exposing them to the air on a substrate having a gate wiring formed thereon, the semiconductor film is then crystallized by irradiation through a protective film with infrared light or ultraviolet light (laser light) and, thereafter, impurities are doped through the protective film to form a source region and a drain region. This impurity doping is made on the semiconductor film through an insulating film (the protective film) covering thereof. It is preferable to sequentially form the gate insulating film, the semiconductor film and the protective film on the substrate having the gate wiring formed thereon without exposing them to the air. The protective film may be formed by irradiating the semiconductor film with laser light.
The present invention adopts the construction in which upon formation of a TFT having the bottom gate structure (typically, the reversed stagger structure), a part of an active layer, at least a channel formation region thereof avoids exposure to the air by the use of the single same chamber, or with employment of a multi-chamber apparatus (examples of which are shown in FIGS. 13 and 14). Such construction may prevent contamination of the active layer interface and realize stable and good electrical characteristics.
According to a first structure of the present invention disclosed in this specification, there is provided a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element comprising:
a gate wiring on a substrate;
a gate insulating film that is in contact with the gate wiring;
an active layer that is in contact with the upper surface of the gate insulating film; and
a protective film that is in contact with the upper surface of the active layer, characterized in that the protective film covers at least a part of a source region, a drain region and a channel formation region formed between the source region and the drain region, which constitute the active layer.
In the above structure of the present invention, the element is characterized in that an end face of the active layer is flush with an end face of the protective film.
In the above structure of the present invention, the element is characterized in that the active layer and the protective film are patterned to have the same shape.
In the above structure of the present invention, the element is characterized in that the active layer is a crystalline semiconductor film that is formed through formation steps including at least a step of irradiating an initial semiconductor film with infrared light or ultraviolet light.
In the above structure of the present invention, the element is characterized in that the active layer is a crystalline semiconductor film that is formed through formation steps including at least a step of crystallizing an initial semiconductor film by irradiating the film through the protective film with infrared light or ultraviolet light.
In the above structure of the present invention, the element is characterized in that the gate insulating film, the initial semiconductor film and the protective film are formed through formation steps including at least a step of sequentially forming and layering each film without exposing them to the air.
In the step of forming and layering the films above, the element is characterized in that the gate insulating film, the initial semiconductor film and the protective film are formed respectively using chambers different from one another.
In the step of forming and layering the films above, the element is characterized in that the gate insulating film and the protective film are formed using a first chamber, and the initial semiconductor film is formed using a second chamber.
In the above structure of the present invention, the element is characterized in that the concentration of boron is 3xc3x971017 atoms/cm3 or less in the interface between the gate insulating film and the channel formation region, or in the interface between the protective film and the channel formation region.
In the above structure of the present invention, the element is characterized in that the concentration of oxygen is 2xc3x971019 atoms/cm3 or less in the interface between the gate insulating film and the channel formation region, or in the interface between the protective film and the channel formation region.
In the above structure of the present invention, the element is characterized in that the concentration of carbon or nitrogen is 5xc3x971018 atoms/cm3 or less in the interface between the gate insulating film and the channel formation region, or in the interface between the protective film and the channel formation region.
In the above structure of the present invention, the element is characterized in that the gate wiring has a single-layer structure or a lamination structure, and is made of a material containing as its main ingredient one element or plural kinds of elements forming a compound, the element or elements being selected from a group consisting of aluminum, tantalum, molybdenum, titanium, chromium and silicon.
In the above structure of the present invention, the element is characterized in that the protective film has a film thickness of 5 to 50 nm.
In this specification, the term xe2x80x9cinitial semiconductor filmxe2x80x9d is a generical name for semiconductor films, and denotes typically a semiconductor film having an amorphous portion, e.g., an amorphous semiconductor film (such as an amorphous silicon film), an amorphous semiconductor film having microcrystal and a microcrystalline semiconductor film. Those semiconductor films are made of Si films, Ge films or compound semiconductor films [for example, amorphous silicon germanium films expressed as SixGe1-x(0 less than X less than 1)]. The initial semiconductor film may be formed by a known method such as the low pressure CVD, the thermal CVD and the PCVD.
In this specification, the term xe2x80x9ccrystalline semiconductor filmxe2x80x9d denotes a single crystal semiconductor film and a semiconductor film containing a crystal grain boundary (which includes a polycrystalline semiconductor film and a microcrystalline semiconductor film), and is used to clearly distinguish those films from a semiconductor film that is amorphous all over its area (an amorphous semiconductor film). Needless to say, what mentioned simply as xe2x80x9csemiconductor filmxe2x80x9d in this specification includes an amorphous semiconductor film in addition to a crystalline semiconductor film.
Also, the term xe2x80x9csemiconductor elementxe2x80x9d in this specification designates a switching element and a memory element, for example, a thin film transistor (TFT), a thin film diode (TFD) or the like.
The present invention has another feature that a mask (such as a resist mask) is formed on the protective film to form an LDD region, which is subsequently patterned.
According to the present invention, there is provided a first method of manufacturing a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element, comprising:
a first step of sequentially forming and layering, on a substrate having a gate wiring formed thereon, a gate insulating film, an initial semiconductor film and an insulating film without exposing them to the air;
a second step of crystallizing the initial semiconductor film by irradiating the film through the insulating film with infrared light or ultraviolet light to obtain a crystalline semiconductor film;
a third step of patterning the crystalline semiconductor film and the insulating film to form a protective film and an active layer an end face of which is flush with an end face of the protective film; and
a fourth step of covering with a mask a region to be a channel formation region of the active layer, and adding through the protective film impurity elements that give N type or P type conductivity.
According to the present invention, there is provided a second method of manufacturing a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element, comprising:
a first step of sequentially forming and layering, on a substrate having a gate wiring formed thereon, a gate insulating film, an initial semiconductor film and an insulating film without exposing them to the air;
a second step of crystallizing the initial semiconductor film by irradiating the film through the insulating film with infrared light or ultraviolet light to obtain a crystalline semiconductor film;
a third step of covering with a mask a region to be a channel formation region of the crystalline semiconductor film, and adding through the insulating film impurity elements that give N type or P type conductivity;
a fourth step of patterning the insulating film to form a protective film;
and
a fifth step of patterning the crystalline semiconductor film to form an active layer an end face of which is flush with an end face of the protective film.
According to the present invention, there is provided a third method of manufacturing a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element, comprising:
a first step of sequentially forming and layering, on a substrate having a gate wiring formed thereon, a gate insulating film, an initial semiconductor film and an insulating film without exposing them to the air;
a second step of crystallizing the initial semiconductor film by irradiating the film through the insulating film with infrared light or ultraviolet light to obtain a crystalline semiconductor film;
a third step of patterning the crystalline semiconductor film and the insulating film to form a protective film and an active layer an end face of which is flush with an end face of the protective film;
a fourth step of covering with a first mask a region to be a channel formation region of the active layer, and adding through the protective film impurity elements that give N type or P type conductivity; and
a fifth step of adding with the use of a second mask impurity elements that give N type or P type conductivity in a region to be a source region or a drain region of the active layer.
According to the present invention, there is provided a fourth method of manufacturing a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element, comprising:
a first step of sequentially forming and layering, on a substrate having a gate wiring formed thereon, a gate insulating film, an initial semiconductor film and an insulating film without exposing them to the air;
a second step of crystallizing the initial semiconductor film by irradiating the film through the insulating film with infrared light or ultraviolet light to obtain a crystalline semiconductor film;
a third step of patterning the crystalline semiconductor film and the insulating film to form a protective film and an active layer an end face of which is flush with an end face of the protective film;
a fourth step of covering with a first mask a region to be a channel formation region of the active layer, and adding through the protective film impurity elements that give N type or P type conductivity;
a fifth step of adding with the use of a second mask impurity elements that give N type or P type conductivity in a region to be a source region or a drain region of the active layer; and
a sixth step of removing the first mask and the second mask at once.
According to the present invention, there is provided a fifth method of manufacturing a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element, comprising:
a first step of sequentially forming and layering, on a substrate having a gate wiring formed thereon, a gate insulating film, an initial semiconductor film and an insulating film without exposing them to the air;
a second step of crystallizing the initial semiconductor film by irradiating the film through the insulating film with infrared light or ultraviolet light to obtain a crystalline semiconductor film;
a third step of covering with a first mask a region to be a channel formation region of the crystalline semiconductor film, and adding through the insulating film impurity elements that give N type or P type conductivity;
a fourth step of adding with the use of a second mask impurity elements that give N type or P type conductivity in a region to be a source region or a drain region of the crystalline semiconductor film;
a fifth step of patterning the insulating film to form a protective film;
and
a sixth step of patterning the crystalline semiconductor film to form an active layer an end face of which is flush with an end face of the protective film.
According to the present invention, there is provided a sixth method of manufacturing a semiconductor device provided with a semiconductor circuit consisting of a semiconductor element, comprising:
a first step of sequentially forming and layering, on a substrate having a gate wiring formed thereon, a gate insulating film, an initial semiconductor film and an insulating film without exposing them to the air;
a second step of crystallizing the initial semiconductor film by irradiating the film through the insulating film with infrared light or ultraviolet light to obtain a crystalline semiconductor film;
a third step of covering with a first mask a region to be a channel formation region of the crystalline semiconductor film, and adding through the insulating film impurity elements that give N type or P type conductivity;
a fourth step of adding with the use of a second mask impurity elements that give N type or P type conductivity in a region to be a source region or a drain region of the crystalline semiconductor film;
a fifth step of removing the first mask and the second mask at once;
a sixth step of patterning the insulating film to form a protective film;
and
a seventh step of patterning the crystalline semiconductor film to form an active layer an end face of which is flush with an end face of the protective film.
In the third to sixth methods of manufacturing a semiconductor device according to the present invention, the method is characterized in that the first mask is a resist mask formed by irradiating the substrate from its back surface with light.
In the above respective manufacturing methods, the method is characterized in that the gate insulating film, the initial semiconductor film and the protective film are formed respectively using chambers different from one another.
In the above respective manufacturing methods, the method is characterized in that the gate insulating film and the protective film are formed using a first chamber, and the initial semiconductor film is formed using a second chamber.
In the above respective manufacturing methods, the method is characterized by further comprising a step of forming as the gate insulating film a laminated film including, among other layers, one layer of a silicon nitride film.
In the above respective manufacturing methods, the method is characterized by further comprising a step of forming as the gate insulating film a laminated film including, among other layers, one layer of a BCB (benzocyclobutene) film.